Radio communicating system



E. 'LABIN ET AL RADIO COMMUNICATING SYSTEM .Wil 26, 1949.

original FiledIJan. 8, 1944 3 Sheets-Sheet 1 Original Filed Jan. 8, 1944 MANS/wwf@ PULSE WA4/5S /Pfcvfo PULSE WAVES E. LABlN ET AL RADIO COMMUNICATING SYSTEM s 'sheets-sheet r el ri OUTPI/ 7' ATTO NEY April 26, 1949f E. LABIN ET AL 2,468,084

RADIO GOMMUNICATING SYSTEM original 4Filed Jan.. 8, 1944 s sheets-sheet s c7/@gi NJW. MN

Patented Apr. 26, 1949 UNITED RADI COMMUNICATING SYSTEM Original application -llanuary 8, 1944, Serial No. 517,478. Divided and this application March 5, 1945, Serial No. 581,102

(Cl. Z50-6) 2 Claims.

This invention is a division of our co-pending application for Radio locating and communicating systems, Serial No. 517,478, i'lled January 8, 1944.

An object of the present invention is to provide a transmitter-receiver system for transmitting and receiving pulse signals of intelligence which are diflicult for unauthorized persons to detect or jam.

The above and other objects ancillary thereto will become more apparent upon consideration of the following detailed description of an embodiment of the invention when read in connection with the accompanying drawings, in which:

Fig. 1 is a schematic diagram of a radio system incorporating a transmitter-receiver arrangement of this invention;

Fig. 2 is a graphical illustration useful for eX- plaining the operating characteristics of the signalling features of the system of Fig. 1;

Fig. 3 is a schematic diagram of a transmitterreceiver circuit that may be used for communication with the system of Fig. 1; and

Fig. 4 is a graphical illustration useful for eX- plaining the receiving operation of the systems of Figs. 1 and 3.

Referring to Fig. 1 a radio signalling system Il) is shown which may comprise a ground station or may be mounted on a vehicle or placed aboard an airplane or ship, as may be desired. In the illustration, the system lll may be regarded as a ground station for detecting aircraft such as airplane l2. If the craft is friendly, it will be provided with a transmitter-receiver device I3', such as shown in Fig. 3, with which communication may be had from the system IB, although it will be understood that the transmitter-receiver or the receiver portion thereof may constitute another station.

The system l includes an antenna lll inductively coupled at I6 to a coil L of a radio frequency oscillator circuit l which includes a vacuum tube 20. The oscillator may be of any triggerable character which when triggered operates to produce a short period of radio frequency waves defining preferably a narrow width pulse for transmission purposes.

The form of oscillating circuit l5 chosen for purposes of illustrating the principles of this invention is of the positive feedback type. Coupled across the terminals of the coil L is the usual tuning condenser C by which the circuit is tunable over a selected band of radio and ultra high frequencies. The L. C. circuit is `connected in the grid-to-cathode circuit of the tube 20 through a blocking condenser 22 which may be adjustable as indicated in the drawing. A grid leak 23 is connected between the grid connection 24 and the grounded cathode connection 26. The plate circuit 25 includes a feedback coil 2l, a radio frequency choke 28 and a load resistor 29 connected to a positive source of plate current. rl`he output connection to of the plate circuit 26 is connected to a demodulator unit 3| which includes a harmonic generator 32, a demodulatc-r 33, a filter 34 and earphones 3d, the details of which are hereinafter described in connection with Fig. 3.

The capacity of the condenser 22 and the resistance of the grid leak 23 are so selected as to provide the desired time constant which determines, in conjunction with the bias applied to the grid connection 2li over resisto-r 36, the cyclic blocking operation of the oscillator circuit. The resistor 3S is normally connected through a second resistor 3l and switch contacts 38, 39 to ground, whereby the grid connection 24 is provided with zero bias. During zero bias the oscillator l5 has a free running cyclic blocking operation according to the time constants of the values of the elements 22 and 23. The operation of the oscillator circuit when controlled by zero bias, is particularly sensitive for reception of messages.

As shown in Fig. l the switch contact 38 is ganged to switch contact 40 which when moved to the position shown places contact 38 in engagement with contact 4I for application of a given negative bias to the grid connection 24, The contact llt is closed at the same time with contact 42 to provide a source of signal impulses Illa, 42?), etc. (curve a, Fig. 2), to control the triggering operation of the oscillator. Curve b of Fig. 2 represents the grid voltage wave 4l for the oscillator tube 20 indicating the triggering action of impulses 42a., 42h, etc. Curve c represents the pulse wave output of the oscillator circuit as transmitted over antenna I4. The pulse waves 52a, 52h, etc., of curve c, correspond in time position to the input impulses 42a, 42h, etc., b-ut represent high power R. F. waves as compared to the low voltage of the input impulses,

It will be noted that the grid voltage of curve 41 follows generally the pattern of a saw-tooth wave wherein the slope of the rising portions 41a. is determined by the time constant of the capacitance 22 and resistance 23 while the portions Mb represent the discharge of the capacitance 22 during which the circuit -oscillates to produce radio or ultra high frequency oscillatory waves` dening the pulse waves 52a, 52h, etc. The broken line 41a represents the triggering level produced by the negative bias at contact 4I at which the oscillator spills over and starts the oscillating period. As indicated by the curve 41 and the pulse waves of curve c, the oscillator operates at spaced intervals to produce pulse waves at a timing dependent upon the occurrence of the signal impulses 42a., 42h, etc. In the absence of the signal impulses the build-up portion 41a of the curve may eventually trigger the oscillator, depending, of course, on the amount of bias at contact lil. For transmitting purposes it is preferable to provide suicient negative bias to avoid self-triggering of the oscillator and, in fact, to render the oscillator insensitive to outside pulse sources. The signal impulses of the system, however, Will be generated of sufcient amplitude to overcome the negative bias and thereby trigger the oscillator according to the time positions of the signal impulses.

For the purpose of communicating with receiver stations capable of detecting time modulated pulses such as the receiver equipment with which a friendly craft would be provided, the signal impulse source of the system shown in Fig. 1 includes, preferably, an impulse generating and modulating system 69 of the cusper type. Electrical current variations produced by speech or other intelligence at microphone 6I may be switched in by opening microphone switch 62 to amplifier 63 and applied to the input coils 64, 65 on transformer El. An alternating current wave B6 is also applied to the transformer through primary winding 68. The alternating current wave 66 controls the average impulse repetition rate, the wave being rectied by a fullwave rectifier circuit including vaculnn tubes 'II and 'I2 whereby a cusper wave B9 is produced. The sec ondary coils 'I3 and 'M are connected to the input grids of the tubes 'H and I2 and to voltage sources 'I5 and 'IS which preferably but not necessarily, diier a given amount so as to render the rectifying operation unbalanced by an amount indicated by the displaced axis 18. This offset bias condition of the rectier circuit causes the cusps produced at the 'output connection 'I9 to be grouped in pairs as indicated by the cusps 82a and 82D. The voltage Variations produced by speech at microphone 9| modifies the offset bias on the rectier circuit causing the axis 'I8 to vary in displacement with respect to the Zero axis TI, thereby displacing the cusps 82a, and 82h toward or away from each other in push-pull manner depending upon the signal energy.

The output of the rectier circuit is taken across the plate load 80 and may be applied to a clipper amplifier 84 for clipping at a level 85 to phone BI is translated into time modulation ofY relatively low voltage impulses 42a., 42h, etc., which, in turn, determine the operating periods of the' oscillator I5. The pulse Waves produced by the oscillator provide the' carrier frequency for the intelligence and, at the same time, a su'iiciently high powered pulse envelope for obstacle detection purposes.

Comparing the -circuits of Figs. 1 and 3, it will be observedv that the two circuits are substantially identical. In Fig. 1 the details of the signal impulse generator and modulator circuit 60 is shown schematically while in Fig. 3 it is shown by block diagram. The demodulator unit 3| is shown in block diagram in Fig. 1 and in schematic form in Fig. 3.

Referring to Fig. 3, the demodulator unit 3| includes two input connections 9| and 92, connection 9| going to a coupling amplifier tube 94 and connection 92 to a demodulator tube 93 through amplier m3. The output connection 95 of the tube 94 is applied to a resonant circuit 9B which is tuned to a desired harmonic. The pulses `from the tube 94 shock excite the circuit 96 into resonance at the tuned harmonic which, preferably, is an odd harmonic of the normal timing characteristic of the pulse waves triggering the oscillator I5.

A phase shifter circuit 98 having a coil 99 coupled with the inductance coil 9'I receives the harmonic energy from the circuit 96 for application to screen grid of the demodulator or mixer tube 93. The grid Il is provided with a negative bias by connection |92 to the mid tap of coil 99. rlhe two ends of the coil 99 are connected to condenser lt and resistor I5, respectively, the relative adjustment of which controls in known manner the phase of the harmonic wave. For further understanding of the ramications of the above-described demodulating principles, reference may be had to the copending application of D. D. Grieg, Serial No. 459,959, filed September 28, 1942, now Patent No. 2,416,306, granted February 25, 1947.

In Fig. 4 curve f represents the pulse waves received by the circuit of Fig. 3 from the system of Fig. 1. For reception the switches 38a and 69a are preferably moved to the positions shown in Fig. 3 whereby the oscillator I5 is provided with zero bias or at least less negative bias than the bias provided at contact 4I. This decreasedl bias lowers the triggering level as shown by broken line lIb as compared with the triggering level lla (see curves b and g, Figs. 2 and 4, respectively). The pulse waves 52a, 52h, etc., of Fig. 4 are time displaced according to a linearly increasing signal input voltage, the broken line positions of the pulse waves indicating the normal timing of the pulse waves in the absence of a modulating signal. The grid voltage wave of curve g indicates the triggering operation of the received pulse waves and curve h represents the' plate output pulse energy at connection 39; These output pulses 92a, 92h, etc., are applied through an amplifier |98 and thence to control grid I I9 of the tube S9. The resulting grid potential produced in the tube 93 by the inputs on grid elements Idil and IIE) is indicated by curve i. The wave I I2 represents the odd harmonic applied to the grid element I @Il and the pulse additions thereon represent the output pulses 92a, 92h, etc. The positions of the pulses on the curve II2 are determined by the degree of time modulation of the pulses. For zero modulation as represented by input pulse wave 5221, a given grid voltage condition is produced. The time displacement, either negatively or positively as determined by the input pulse waves, controls the position o'f the pulses on the slope of curve M2. Thus, for displacement due to a negative polarity of the input pulse wave 52a, the corresponding pulse 92a assumes a lower position than pulse 92h. For input pulse waves having a displacement of progressively greater positive polarity, such as pulse Waves 52o, 52d, etc., the corresponding pulses 92e, 92d, etc., occur at progressively higher points on the curve I I2.

The bias on the control grid IIO of the tube 93 is selected so as to provide a threshold clipping level H4 which clears the amplitude peaks of the wave H2. Thus, the output of the tube 93 comprises a train of pulses 93a, 93h, etc., modulated in amplitude according to the time displacement of the corresponding input pulse waves. These pulses define an envelope II5 which corresponds to the signal energy at the transmitting station. By applying the amplitude modulated pulses oi' curve :i to a suitable lter |29, the signal wave represented by envelope I I5 is detectable on earphones |22.

In order to reply from the craft I2, the switches 39a and 40a are changed to the condition represented by the switch contacts 38 and t@ of Fig. l. The transmitter-receiver circuit I3 is then conditioned for transmission similarly as hereinbefore described in connection with the transmitter circuit portion of Fig. 1. The operation is the same and need not be repeated. The system I9, however, will have the switch contacts 39 and 40 shifted to the positions indicated by contacts 38a and 49a of Fig. 3, whereby the system l0 will be sensitive to the pulse waves received from the circuit I3.

It will be readily apparent from the foregoing description that the triggering principle of the invention renders the circuit substantially free from interference and jamming. Interference pulses occuring during the low portion of the grid voltages as indicated by curves b and g will not trigger the circuits unless they are of exceptionally high power. Thus, for ordinary interfering pulses to trigger the circuit they must occur near or substantially in coincidence with the timing of the pulse waves. Further, since interfering pulses are usually at a repetition rate diierent from the repetition rate of a given train of pulse waves, they will not occur in substantial coincidence with the pulse waves except at widely spaced intervals thereby producing, in eiect, a beat frequency interference. It will also be observed that by increasing the negative bias on the circuit during transmission, the increased bias reduces the sensitivity of the circuit to outside pulses thereby substantially eliminating all interference whether produced by jamming attempts or by another similar transmitter-receiver circuit.

It will also be clear that our system includes a new method and means for transmitting high power time modulated pulse waves of a given radio or ultra high frequency in response to voltage impulses of low power. This is of particular advantage over pulse wave transmitting systems heretofore proposed since no high power modulating signals are required, the modulation being eiected in our system by simple synchronization of the triggering operation of the carrier frequency oscillator.

While we have described above the principles of our invention in connection with specific appa ratus, it is to be clearly understoodthat this de- -scription is made only by way of example and not as a limitation on the scope of our invention as set forth in the objects of our invention.

We claim:

1. A transmitter-receiver circuit comprising a. triggerable oscillator circuit, control means for determining the periodic pulsing operation of the oscillator for producing recurring pulse waves, detector means coupled with said oscillator circuit to detect time variations in the periodic operation thereof, means for producing a train of signal time modulated impulses, means to apply said signal impulses to said control means for triggering of said oscillator, thereby synchronizing the timing of the periodic operation thereof to the time positions of said time modulated impulses, means to selectively apply two different bias potentials to said control means for selection of oscillator operation between two different operating conditions, one of said conditions causing the oscillator to have a pulsing operation of a given timing for sensitive triggering operation in response to received signal pulses of attenuated ampltude occurring at a time spacing wherein the maximum time interval between successive signal pulses is no greater than the period of said given timing, the other of said conditions caus ing said oscillator to have ia pulsing operation the period of which is much greater than the period of said given timing thereby rendering the oscillator less sensitive to reception of said attenuated signal pulses but suiiiciently sensitive to be triggered by locally generated signal impulses for transmission purposes.

2. A transmitter-receiver circuit comprising a triggerable oscillator circuit, control means for determining the periodic pulsing operation of the oscillator for producing recurring pulse waves, detector means coupled with said oscillator circuit to detect time variations in the periodic operation thereof, means for producing a train of signal time modulated impulses, means to apply said signal impulses to said control means for triggering of said oscillator, thereby synchronizing the timing of the periodic operation thereof to the time positions of said time modulated impulses, said detector means including means responsive to the operating periods of the oscillator to generate a selected harmonic wave, means to control the phase of said harmonic Wave, and mixer-clipper means to combine the output pulses of the oscillator with said harmonic wave, whereby signal energy which is amplitude modulated according to the time displacement of said pulse Waves is detectable.

EMILE LABIN. DONALD D. GRIEG. ARNOLD M. LEVINE.

REFERENCES CITED The following references are of record in the ille of this patent:

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